Air Pollution

The 1970 Clean Air Act in the United States recognized seven air pollutants as being in immediate need of regulatory monitoring. These pollutants are sulfur dioxide, particulates (such as dust and smoke), carbon monoxide, volatile organic compounds, nitrogen oxides, ozone, and lead. These pollutants were regarded as the greatest danger to human health. Because criteria were established to limit their emission, these materials are sometimes referred to as "criteria pollutants." Major revisions to the Clean Air Act in 1990 added another 189 volatile chemical compounds from more than 250 sources to the list of regulated air pollutants in the United States.

Some major pollutants are not directly poisonous but can harm the environment over a longer period of time. Excess nitrogen from fertilizer use and burning of fossil fuels is causing widespread damage to both aquatic and terrestrial ecosystems on Earth's surface. For example, over-fertilizing of plants favors the growth of weedy species. Pollutants can also damage the atmosphere above Earth's surface. A well-known example of this damage is that caused by chlorofluorocarbons (CFCs). CFCs were used for many years as coolant in refrigerators and as cleaning agents. While generally chemically inert and non-toxic in these settings, CFCs diffuse into the upper atmosphere where they destroy the ultraviolet-absorbing ozone shield. Ozone depletion is a concern for the health of humans, as increased exposure to the sun's ultraviolet radiation can cause genetic damage that is associated with various cancers, especially skin cancer.

Air pollutants can travel surprisingly far and fast. About half of the fine reddish dust visible in Miami's air during the summer is blown across the Atlantic Ocean from the Sahara Desert. Radioactive fallout from an explosion at the Chernobyl nuclear reactor in the Ukraine was detected many miles away in Sweden within two days after its release and spread around the globe in less than a week.

One of the best-known examples of long-range transport of air pollutants is acid rain. The acids of greatest concern in air are sulfuric and nitric acids, which are formed as secondary pollutants from sulfur dioxide and nitrogen oxides released by burning fossil fuels and industrial processes such as smelting ores. These acids can change the pH (a standard measure of the hydrogen ion concentration or acidity) of rain or snow from its normal, near neutral condition to an acidity that is similar to that of lemon juice. Although this acidity is not directly dangerous to humans, it damages building materials and can be lethal to sensitive aquatic organisms such as salamanders, frogs, and fish. Thousands of lakes in eastern Quebec, New England, and Scandinavia have been acidified to the extent that they no longer support game fish populations. Acid precipitation has also been implicated in forest deaths in northern Europe, eastern North America, and other places where air currents carry urban industrial pollutants.